Ink jet printers commonly include one or more printheads which are mounted on a carriage assembly. The carriage assembly is moveable in a transverse direction (generally perpendicular) relative to an advance direction of a print medium such as paper. As the printhead moves across the surface of the print medium during a particular pass, ink is selectively ejected from a plurality of nozzles pursuant to commands from a microcomputer or other controller and deposited on the print medium at corresponding ink dot placement locations, called pixels, in the image of the print medium. The image created from individual ink dots on the printing medium result in a two-dimensional pixel array, which can be referred to as the image bitmap.
Many conventional high-quality inkjet printers use a swath-by-swath approach. The band of colored ink drops printed at a time is generally referred to as the “swath”. Typically, four different color inks (cyan, magenta, yellow, and black) are used by the printer to print the range of colors contained in the image. By printing successive swaths, the image is completely formed on the print medium. Each swath extends across the entire width of the image bitmap, e.g., corresponding to the printed image area across a page. The vertical size of the swath is at most the size of the printhead, but it can be smaller, as described more in detail in the following.
Typically, ink jet printers use a variety of different print modes to control the quality and speed of printing. In one pass printing, each horizontal motion of the printhead across the print medium is used to print a portion of the image bitmap in one single scan. This print mode is fastest, but it can result in print quality defects, particularly at swath boundaries. Such defects appear periodically on the printed image and they are commonly known as “banding defects”.
To improve print uniformity, multi-pass printing mode is employed by printing overlapping swaths having a partial printing density in a process known as “shingling.” During the typical two-pass shingled printing, each swath prints only every other dot in a final image line, and the resulting fully printed image bitmap will have been printed with ink contributed by two passes of the print head. Three- or four-pass shingling follows the same principle, providing even better print quality with slower printing speed. In multi-pass printing mode, the distance between the top scan line of a swath and the swath following is a distance less than the height of the swath.
As the printing mode may have a number of parameters, such as the number of passes required to fill an image area and the enabled position of the ink droplets at every pass, i.e., the pixel locations to be filled, a digital matrix is generally created to define each position of each pass in which a drop may print. The matrix is called the print mask. The print mask then determines exactly which pixels are printed and which are not on each pass of the print head.
For instance, in four-pass printing mode, a ¼ filled printing mask can be employed in each pass with an advancement of ¼ of the swath length after each pass.
U.S. Pat. No. 6,764,162 describes a method for reducing inkjet printer banding effects which provides a shingle mask that creates a smooth accumulated shingle mask distribution, which will tend to eliminate the sharp transition regions that create the banding characteristics of inkjet printers.
In some types of printheads, it is common that nozzles located at the top and the bottom of the printhead exhibit significant dot placement errors in the advancement direction if compared to the nozzles at the central portion of the printhead. A multi-pass printing mode can be insufficient to improve print quality to an acceptable level, especially when specific groups of nozzles have worse errors than other groups of nozzles.
U.S. Pat. No. 6,310,640 discloses a system including a print controller which activates the nozzles to deposit the ink onto a medium during multiple printing passes, as governed by a printmask. The printmask has a mask pattern that allows some of the nozzles to deposit drops of the ink in fewer possible pixel locations of the row in each printing pass, and allowing others of the nozzles to deposit the drops of the ink in more possible pixel locations of the corresponding ones of the rows in each printing pass. The fewer and more locations are relative to printing a substantially equal number of possible pixel locations on a row with all nozzles.
U.S. Pat. No. 6,375,307 discloses an inkjet printing apparatus having a printhead comprising a plurality of nozzles arranged along a printhead axis, the printing apparatus being arranged to print swaths which overlap the neighbouring swaths by a fraction of the swath width, wherein only one edge region of each swath which overlaps the neighbouring swaths has a reduced print density relative to its remaining regions.